The present invention relates to a mixing head for an RIM molding machine which mixes plural kinds of liquid materials such as urethane resin, epoxy resin and polyamide resin.
The present invention is an improvement of a mixing head disclosed in the co-pending U.S. Patent Application No. 07/162,206. FIG. 6 shows such a mixing head. In the mixing head, two raw liquid materials of a polyol A and an isocyanate B are mixed with each other in a mixing chamber 3 and the mixture of the materials is discharged to a sprue gate 9a of a molding die 9. The mixing head 1 has a normally-closed discharge control valve 16 for opening and closing inflow passages 4A and 4B for the liquid materials A and B to the mixing chamber. The valve 16 comprises a valve shaft 15 which is moved back and forth in the mixing chamber 3, and a first piston 14 which is coupled with the valve shaft and is moved back and forth in a first cylinder 11 provided in a body 2 of the mixing head 1.
The mixing head 1 also has a normally-open circulation control valve 26 for opening and closing return passages 5A and 5B ramified from the inflow passages 4A and 4B. The valve 26 comprises a spool 25 having annular grooves 25a and 25b for opening and closing the return passages 5A and 5B, and a second piston 24 which is coupled with the spool and is moved back and forth in a second cylinder 21 provided in the body 2 of the mixing head 1.
A right-hand chamber (as to FIG. 6) 11a of the first cylinder 11 communicates with a first forward movement passage 12 for a working oil 0 to move the first piston 14 leftward (as to FIG. 6) to open the discharge control valve 16. A left-hand chamber 11b of the first cylinder 11 communicates with a first backward movement passage 13 for the working oil 0 to move the first piston 14 rightward to close the discharge control valve 16.
A right-hand chamber 21a of the second cylinder 21 communicates with a second forward movement passage 22 for the working oil 0 to move the second piston 24 leftward to close the circulation control valve 26. A left-hand chamber 21b of the second cylinder 21 communicates with a second backward movement passage 23 for the working oil 0 to move the second piston 24 rightward to open the circulation control valve 26.
The first and the second forward movement passages 12 and 22 communicate with each other and are connected to a hydraulic pump 7 and an oil tank 8 through a four-port two-position electromagnetic changeover valve 6. The first and the second backward movement passages 13 and 23 communicate with each other and are connected to the hydraulic pump 7 and the oil tank 8 through the changeover valve 6.
When the mixture M of the materials A and B is to be supplied from the mixing head 1 to the molding die 9, the changeover valve 6 is put in an operational position as shown in FIG. 7. As a result, the pressure of the working oil 0 acts to the right-hand chambers 11a and 21a of the first and the second cylinders 11 and 21 through the first and the second forward movement passages 12 and 22 so that the first and the second pistons 14 and 24 are moved leftward (as to FIG. 7) to open the discharge control valve 16 and close the circulation control valve 26. At that time, the valve shaft 15 of the valve 16 is moved back so that the mixing chamber 3 is formed, the inflow passages 4A and 4B are opened, the liquid materials A and B are mixed with each other in the mixing chamber, and the mixture M of the materials is supplied from the mixing head 1 to the molding die 9.
When the supply of the mixture M from the mixing head 1 to the molding die 9 is to be stopped, the changeover valve 6 is put in the original position thereof as shown in FIG. 6. As a result, the pressure of the working oil 0 acts to the left-hand chambers 11b and 21b of the first and the second cylinders 11 and 21 through the backward movement passages 13 and 23 so that the first and the second pistons 14 and 24 are moved rightward to close the discharge control valve 16 and open the circulation control valve 26. At that time, the valve shaft 15 of the discharge control valve 16 is moved forth so that the inflow passages 4A and 4B are closed, the return passages 5A and 5B are opened, the liquid materials A and B circulate through the passages therefor, respectively, and the supply of the mixture M from the mixing head 1 is stopped.
However, an extraneous substance such as hardened materials of the raw liquid materials A and B or the mixture M clings to the outside circumferential surfaces of the valve shaft 15 and the spool 25 so that the valve shaft and the spool are likely to be hindered from being smoothly moved back and forth.
When the spool 25 is moved later than the valve shaft 15 due to the hindrance at the beginning of the supply of the mixture M from the mixing head 1 to the molding die 9, the inflow passages 4A and 4B are opened while the return passages 5A and 5B remain opened, so that the pressure of mixing the liquid materials A and B in the mixing chamber 3 falls. At that time, the liquid materials A and B are not well mixed with each other, due to the fall in the pressure and the mixture M not sufficiently mixed, is supplied to the molding die 9. Accordingly, the molded product of the mixture M becomes defective.